1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
In a typical image forming apparatus employing a serial head system (a printing system in which a printing is performed by moving a carriage including an ink-jet head in the main scanning direction) while conveying a sheet in the sub-scanning direction by sticking the sheet on a surface of a conveying belt by an electrostatic force to print an image on a sheet, a leading end portion of the sheet or lateral end portions of the sheet is detected with a reflection-type photosensor mounted on the carriage, thereby determining a print area of the sheet accurately. Therefore, the image forming apparatus can accurately prints the image on the sheet within the print area, and thus it is possible to prevent the conveying belt from being stained with ink.
If the image forming apparatus fails to print the image on the sheet within the print area, a portion of the image is not printed on the sheet or the image is not printed on a specified area of the sheet, which resulting in a print error.
When a portion of the image is not printed on the sheet, an image corresponding to the portion is printed on the conveying belt, so that the conveying belt is stained with ink. The conveying belt is generally made of a very thin material, so that on a portion of which is stained with the ink, the electrostatic force acting on the sheet is reduced. As a result, the sheet is lifted up from the conveying belt. Consequently, it may cause such problems that a printed image is blurred or the print head is damaged because a surface of the sheet is rubbed against the print head due to a floating of the sheet.
To cope with the problems, for example, Japanese Patent Application Laid-open No. 2006-038639 discloses an end position detecting device. The end position detecting device accurately detects a position of an end portion of a sheet based on data on a reflected light detected by a reflection-type photosensor without being affected by a color variation of a surface of the sheet or a surface roughness of the sheet. The data on the reflected light includes data on a light reflected on a supporting member and data on a light reflected on the sheet supported by the supporting member.
However, for example, when an image is printed on a sheet with a printed pattern (hereinafter, “a pattern-printed sheet”), such as company stationery with a company logo, a detection accuracy of the end position detecting device disclosed in Japanese Patent Application Laid-open No. 2006-038639 becomes inaccurate because a false detection of the photosensor is induced. For example, when a pattern printed on the substantially center of a pattern-printed sheet is detected as an end portion of the pattern-printed sheet, it is not possible to print an image on a specified area of the pattern-printed sheet properly.